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The Pacific Journal of Science and Technology –562– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
Geoelectric Sounding to Delineate Shallow Aquifers in the Coastal Plain Sands of Okitipupa Area, Southwestern Nigeria.
G.O. Omosuyi, J.S. Ojo, and M.O. Olorunfemi*
Department of Applied Geophysics, Federal University of Technology,
PMB 704, Akure, Ondo State, Nigeria.
*Department of Geology, Obafemi Awolowo University, Ile-Ife, Nigeria.
ABSTRACT In Okitipupa area, south western Nigeria, the aquifers of the Coastal Plain sands are largely Quaternary porous sediments, but without records of their subsurface disposition. In this study, fifty Schlumberger soundings were conducted over the study area, aimed at delineating the boundaries, depth range of the shallow aquifer units, and assessing their vulnerability to near-surface contaminants. The study delineated two major aquifer units within the area: (i) the upper/surficial aquifer system, which occur at depths ranging from 5.8m (around Agbabu) to 61.5m (around Ikoya), and with materials of higher average resistivity (504.7 Ωm), suggestive of gravelly/coarse to medium-grained sand and (ii) the intermediate aquifer system, characterised by depth range of 32.1-127.5m, average resistivity of 296.8Ωm, typical of medium-grained sand saturated with water. The highly resistive, impermeable materials overlying the aquifer units around Ajagba, Aiyesan, Agbetu, Ilutitun, Igbotako, and Erinj suggests that the aquifer units are less vulnerable to near-surface contaminants than in Agbabu, Igbisin, Ugbo, and Aboto where aquifers are overlain by less resistive materials. This study enabled the delineation of shallow aquifers, their subsurface disposition, and identified promising areas for elaborate groundwater development in the area. An integration of such geophysical study with lithologic logs/drilling data would enhance accurate delineation of aquifers and vulnerability quantification in the area. (Keywords: geoelectric sounding, aquifer units, coastal
plain sands, aquifer vulnerability)
INTRODUCTION The Okitipupa area is underlain by the sedimentary rocks of south western Nigeria. The zone constitutes the easternmost segment of the extensive Dahomey basin. Studies have shown that sedimentary basins generally contain enormous quantities of water (Freeze and Cherry, 1979; Fetter, 1980; Kehinde and Loehnert, 1989; Edet and Okereke, 2002). To realise this fact, several countries worldwide have engaged in extensive studies of basins within their frontiers, using geological/hydrogeological, geophysical, or chemical data, or a combination of the above, for proper identification and delineation of the water bearing units. Such studies (Leit and Barker, 1978; van Overmeeren 1989; Mbonu et al., 1991; El-Waheidi et al., 1992; Kalinski et al., 1993; Frohlich et al., 1994; Ebraheem et al., 1997; Choudhury et al., 2001; Edet and Okeke, 2002; and Lashkaripour, 2003), often afford the optimisation and proper management of the basins in order to enhance safe discharge and appropriately safeguarding the quality status of the water. However, any rational development groundwater requires geophysical data input, among others. Few studies so far conducted in the area (Ako, 1982; Omosuyi et al., 1999; Omosuyi, 2001) were based on borehole lithologic correlation and hydro-geophysical characterisation of the subsurface sequence for suitable location of groundwater abstraction points. In this study, geoelectric resistivity sounding data have been used to delineate the boundaries, subsurface disposition and assess the vulnerability status of the shallow water bearing aquifers in the Coastal Plain around Okitipupa.
The Pacific Journal of Science and Technology –572–
http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
The study is envisaged to upgrade our knowledge of the basin and enhance the success rate of groundwater abstraction boreholes in the area, in addition to serving as a useful guide to an elaborate groundwater development programme proposed for the area. PHYSIOGRAPHY, GEOLOGY AND HYDROGEOLOGY The study area (Figure 1) is bounded by Longitudes 4
0 22 ‘E and 5
0 05 ‘E, and latitudes 6
0
00’N and 60 40’N. The terrain is regionally gently
undulating southward; topographic elevations vary from about 84m above sea level in the northern part, with gradual slope to a near sea level swamp flat in the coastal area to the south. The area is drained by many perennial streams and rivers among which are Ominla, Oluwa, Akeun, Ufara, and Oni, while the southern part is particularly characterised by lagoons, coastal creeks, canals, and several tributaries to the extensive River Oluwa. The annual temperature ranges from 24 to 27
0C and the mean annual
rainfall is over 2500mm (Iloeje, 1981).
BIGHT OF BENIN
ODIGBO
OKITIPUPA IRELE
ESE - ODO
ILAJE
OmotosoIlebe
LokuboAbusoro
Agbetu
ErekitiIlado
Irowo
IgbotakoIju OkeLadenkehin
Ode AyeSettlement
IgbolouyeForikuMulekangbo
Ilubirin
Agbabu
Igbaje
Oluagbo
IwadaIjuodo
IlutitunIkoya
OlotoMahintedoOde - Igbodigbo
Igboegunrin
Ebute - ErinjeAtijere
Ayesan
Aroromi Obu
Olowo
Lamudifa
OtuMkt
Olorunsogo
Odigbo
OREOfosu
Oja FagboOnisere
OrisunmibareOtugbembo
Poroye CampLodaAyadi
Legbagbo
Ladan Camp
Lejuwa
Laworo
Ebute Ipare
Kurawe
Fishing CampMahinUgbo
Ugbonla
Idi Ogba Ajapa
Oroto
Aiyetoro
Ashisha
Upe
IlepeteObe AduObenla
SekelewuOjumole
Aboto
IGBEKEBO
IGBOKODA
ShabomiIgbotu
OmiIRELEOKITIPUPA
Igodan
Old Ayeka
Araromi - Ayeka
Italuwa
Ago Ukgbene IpokaOjuala
AjodaSalawa
Shegbemi
Gbeleju
Nagbore's Camp
Yopele
Madagbayo
AmapereArogbo
Uba - ArogboFiafio
Saforogbon
Okomu
Okorogbon
Ijaw Camp
Biagbene
Olowoso
Ayande
Gbu
Okoron ado
AkimoUpe
Gbelemotin
Soba Camp
Kajola
oke alafia
Oniparaga
Origbe
Lamira
Arijan
L E G E N D
Major roads
Minor roads
VES Location
Local Councils Capital
Rivers
Footpath
6 00'
6 30'
4 30' 5 00'
10 Km0
Figure 1: Location Map showing Okitipupa Area.
The Pacific Journal of Science and Technology –564– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
The area is underlain by the Coastal Plain Sands or the Benin Formation (Figure 2). The sediments of the Coastal Plain, deposited during the Late Tertiary – Early Quaternary period (Jones and Hockey, 1964), consist of unconsolidated, coarse to medium- fine grained sands and clayey shale in places (Okosun, 1998). The sands are generally moderately sorted and poorly cemented. The Benin Formation is overlain by lateritic overburden or recent alluvial deposits and underlain by the Paleocene Akinbo Formation. This formation is predominantly shally. Outcrops of shales were mapped around a spring at Ode Aye (Omosuyi, 2001). The Akinbo shale is underlain by the continental Cretaceous sediments of the Abeokuta Group (Omatsola and Adegoke, 1981). The Coastal Plain sands constitute the major shallow hydro-geologic units in the area. Aquifers are characteristically continental sands, gravels, or marine sands. The lateritic earth overlying the
sands, as well as the underlying impervious clay/shale member of the Akinbo Formation, constitute protective configuration for the aquifer units. Also, the high annual rainfall and other favourable climatic and geologic factors guarantee adequate groundwater recharge in the area. DATA ACQUISITION AND INTERPRETATION Fifty (50) Schlumberger vertical electrical soundings (VES) were conducted across the study area using a maximum current electrode separation (AB) of 650m. Figure 2 shows the VES locations. Resistivity measurements were made with a digital resistivity meter (PASI – E2 DIGIT) which allows for readout of current (I) and voltage (V).
Figure 2: Geological Map of the Study Area (PTF, 1997), Showing the Sample Locations.
The Pacific Journal of Science and Technology –565– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
The field curves were interpreted through partial curve matching (Koefoed, 1979) with the help of master curves (Orellana and Mooney, 1966) and auxiliary point charts (Zohdy, 1965; Keller and Frischnecht, 1966). From the preliminary interpretation, initial estimates of the resistivity and thickness of the various geoelectric layers at each VES location were obtained. These geoelectric parameters were later used as starting model for a fast computer-assisted interpretation (Vander Velpen, 1988). The program takes the manually derived parameter as a starting geoelectric model, successively improved on it until the error is minimised to an acceptable level. RESULTS AND DISCUSSION Geoelectric and Lithological Characteristics Figure 3 shows typical geoelectric curves corresponding to VES data from the study area. Curve types identified ranges from simple AK, QH, to HKH, HKQ, KHK and complex HAKQ, AKHKQ, etc. types, reflecting facies or lithological variations in the area. The AK, HKH and HKQ types are the most preponderant, with each constituting about 7.9%, while each of QH, KHK, HKHK, HAKQ and QHKQ accounts for about 5.3%. The general signature of the curves suggests alternate sequence of conductive-reflective, resistive-conductive layers, reflecting the unconsolidated nature of the Quaternary coastal plain sequence, characterised by intercalation of sands and clay/shale horizon (Omosuyi et al., 1998). This reflects on the curves as discrete resistivity layers (Examples in Figure 3). Aquifer Delineation Electrical methods primarily reflect variations in ground resistivity. The electrical resistivity contrasts existing between lithological sequences (Dodds and Ivic, 1998; Lashkaripour, 2003) in the subsurface are often adequate to enable the delineation of geoelectric layers and identification of aquiferous or non-aquiferous layers (Schwarz, 1988). The geoelectric sequence suggests a subsurface geology characterised by alternation of sands/gravels, clay/shale and sandstone
occurring at varying depths with variable thicknesses. The sand and gravel layers constitute the aquifer units. The geoelectric parameters of the aquifer units were determined from the interpretation of the sounding curves, assisted by the distinctive resistivity contrasts between the discrete geoelectric layers. The upper and lower aquifer horizons work are referred to as the surficial (upper) and intermediate (lower) aquifers respectively. The results of the interpretation were used to construct three interpretive sections AB, CD, and EF, taken in the north-south, west-east and northwest-southeast directions (Figure 4). The sections (Figures 4a and 4b) reveal that the surficial and intermediate aquifers are overlain in places by materials with variable thickness and resistivity parameters. In section AB (Figure 4a), the resistivity of materials overlying the upper aquifer ranges from 2 Ωm to 5622Ωm. The depth to the top of the aquifer varies from 20.5m to 39m. The high resistivity of the top layers may correspond to the unsaturated zone, as observed in Van Overmeeren (1989). The depth to the intermediate aquifer ranges from 52m to 79.8m. In the area, depths to the top of the aquifer horizons tend to increase southward. The thickness and resistivity parameters of materials overlying an aquifer are important parameters in the assessment of the vulnerability of the underling aquifer (Kalinski et al., 1993). In geoelectric section CD, a west-east cross section (Figure 4b), depths of occurrences range from 16.1 to 48.3m and 28.1 to 96.4m in the upper and intermediate aquifers respectively. The northwest-southeast section reveals that depth to the upper aquifer ranges from 15.5 to 61.5m, while that of the intermediate aquifer varies between 54.7m and 97.5m. The geoelectric sections show that the two aquifer units generally dip southward. Contour maps showing the depth to the top of the aquifer units are shown in Figures 5a and 5b. The isopach maps constructed from the VES results (Figure 6) show that the surficial and intermediate aquifers are highly variable in thickness across the area. In the former, thickness ranges from 5.1m (at Loda) to 49.4m at Erinje, with a mean value of 29.5m.
The Pacific Journal of Science and Technology –566– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
(a)
(b)
Figure 3: Typical VES Curves from Okitipupa Area: (a) Igbotako and (b) Irele.
The Pacific Journal of Science and Technology –567– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
Agbetu
Igbotako
Mahintedo
Erinje
Aboto
34744
1880
5622
455
447 Ohm-m
76
207
496
367
111
215
796
742
628
400
339
234
277
1368
982
SA
435
208 Ohm-m
SA
682
897
203 Ohm-m
55
10
-160
-120
-80
-40
0
Depth (m)
IA
A B
Aiyesan
Ilutitun
OkitipupaIrele
Iyasan
240 694
5000
1336
392 Ohm-m
125
IA
450 958
891
4930
885
205
SA
2920
629
166
149
IA
1548
877
459
714
628
245
184 Ohm-m
SA
435
1056
1055 Ohm-m
515
206
72
-160
-120
-80
-40
0
Depth (m)
C D
Araromi ObuIwada Ikoya Igbodigo
Ojuala
Amapere
597
208
1449
324
1659 Ohm-m
486
282
2073
167
45
595
373
288 Ohm-m
383
122 168
153
281
906
784
1168 Ohm-m
621
81
IA
413
2441
1389
879122
410
159
174
78
IA
SA
-160
-120
-80
-40
0
Depth (m)
E F
Topsoil/cap rock
Sand (Surfacial aquifer)
Sand (Intermediate aquifer)
Resistivity value (Ohm-m)
LEGEND
SA
IA
205
80
40
10 20
m
km
Scale
Figure 4: Geoelectric Section along North-South (AB), West-East(CD), and Northwest-Southeast Directions in the Area.
The Pacific Journal of Science and Technology –568– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
BIGHT OF BENIN
ODIGBO
OKITIPUPA IRELE
ESE - ODO
ILAJE
OmotosoIlebe
LokuboAbusoro
Agbetu
Erekiti
Ilado
Irowo
IgbotakoIju OkeLadenkehin
Ode Aye
Settlement
IgbolouyeForikuMulekangbo
Ilubirin
Agbabu
Igbaje
Oluagbo
IwadaIjuodo
IlutitunIkoyaOlotoMahintedo
Ode - IgbodigboIgboegunrin
Ebute - ErinjeAtijere
Ayesan
Aroromi Obu
Olowo
Lamudifa
OtuMkt
Olorunsogo
Odigbo
OREOfosu
Oja FagboOnisere
OrisunmibareOtugbembo
Poroye CampLodaAyadi
Legbagbo
Ladan Camp
Lejuwa
Laworo
Ebute Ipare
Kurawe
Fishing CampMahinUgbo
Ugbonla
Idi Ogba Ajapa
Oroto
Aiyetoro
Ashisha
Upe
IlepeteObe AduObenla
SekelewuOjumole
Aboto
IGBEKEBO
IGBOKODA
ShabomiIgbotu
OmiIRELEOKITIPUPA
Igodan
Old Ayeka
Araromi - Ayeka
Italuwa
Ago Ukgbene IpokaOjuala
Ajagba Salawa
Shegbemi
Gbeleju
Nagbore's Camp
Yopele
Madagbayo
AmapereArogbo
Uba - ArogboFiafio
Saforogbon
Okomu
Okorogbon
Ijaw Camp
Biagbene
Olowoso
Ayande
Gbu
Okoron ado
AkimoUpe
Gbelemotin
Soba Camp
Kajola
oke alafia
Oniparaga
Origbe
Lamira
Arijan
6 00'
6 30'
4 30' 5 00'
10 Km0
Akata
Iyasan
Erinje
Igbisin Ebute Irele
LEGEND
Major roads
Minor roads
VES Location
Local Councils Capital
Rivers
Footpath
Figure 5a: Depth to the Top of Surficial Aquifer in the Area.
The Pacific Journal of Science and Technology –569– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
BIGHT OF BENIN
ODIGBO
OKITIPUPA IRELE
ESE - ODO
ILAJE
OmotosoIlebe
LokuboAbusoro
Agbetu
Erekiti
Ilado
Irowo
IgbotakoIju OkeLadenkehin
Ode Aye
Settlement
IgbolouyeForikuMulekangbo
Ilubirin
Agbabu
Igbaje
Oluagbo
IwadaIjuodo
IlutitunIkoyaOlotoMahintedo
Ode - IgbodigboIgboegunrin
Ebute - ErinjeAtijere
Ayesan
Aroromi Obu
Olowo
Lamudifa
OtuMkt
Olorunsogo
Odigbo
OREOfosu
Oja FagboOnisere
OrisunmibareOtugbembo
Poroye CampLodaAyadi
Legbagbo
Ladan Camp
Lejuwa
Laworo
Ebute Ipare
Kurawe
Fishing CampMahinUgbo
Ugbonla
Idi Ogba Ajapa
Oroto
Aiyetoro
Ashisha
Upe
IlepeteObe AduObenla
SekelewuOjumole
Aboto
IGBEKEBO
IGBOKODA
ShabomiIgbotu
OmiIRELEOKITIPUPA
Igodan
Old Ayeka
Araromi - Ayeka
Italuwa
Ago Ukgbene IpokaOjuala
Ajagba Salawa
Shegbemi
Gbeleju
Nagbore's Camp
Yopele
Madagbayo
AmapereArogbo
Uba - ArogboFiafio
Saforogbon
Okomu
Okorogbon
Ijaw Camp
Biagbene
Olowoso
Ayande
Gbu
Okoron ado
AkimoUpe
Gbelemotin
Soba Camp
Kajola
oke alafia
Oniparaga
Origbe
Lamira
Arijan
LEGEND
Major roads
Minor roads
VES Location
Local Councils Capital
Rivers
Footpath
6 00'
6 30'
4 30' 5 00'
10 Km0
Akata
Iyasan
Erinje
Igbisin Ebute Irele
Figure 5b: Depth to the Top of Intermediate Aquifer in the Area.
The Pacific Journal of Science and Technology –570– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
BIGHT OF BENIN
ODIGBO
OKITIPUPA IRELE
ESE - ODO
ILAJE
OmotosoIlebe
LokuboAbusoro
Agbetu
Erekiti
Ilado
Irowo
IgbotakoIju OkeLadenkehin
Ode Aye
Settlement
IgbolouyeForikuMulekangbo
Ilubirin
Agbabu
Igbaje
Oluagbo
IwadaIjuodo
IlutitunIkoyaOlotoMahintedo
Ode - IgbodigboIgboegunrin
Ebute - ErinjeAtijere
Ayesan
Aroromi Obu
Olowo
Lamudifa
OtuMkt
Olorunsogo
Odigbo
OREOfosu
Oja FagboOnisere
OrisunmibareOtugbembo
Poroye CampLodaAyadi
Legbagbo
Ladan Camp
Lejuwa
Laworo
Ebute Ipare
Kurawe
Fishing CampMahinUgbo
Ugbonla
Idi Ogba Ajapa
Oroto
Aiyetoro
Ashisha
Upe
IlepeteObe AduObenla
SekelewuOjumole
Aboto
IGBEKEBO
IGBOKODA
ShabomiIgbotu
OmiIRELEOKITIPUPA
Igodan
Old Ayeka
Araromi - Ayeka
Italuwa
Ago Ukgbene IpokaOjuala
Ajagba Salawa
Shegbemi
Gbeleju
Nagbore's Camp
Yopele
Madagbayo
AmapereArogbo
Uba - ArogboFiafio
Saforogbon
Okomu
Okorogbon
Ijaw Camp
Biagbene
Olowoso
Ayande
Gbu
Okoron ado
AkimoUpe
Gbelemotin
Soba Camp
Kajola
oke alafia
Oniparaga
Origbe
Lamira
Arijan
LEGEND
Major roads
Minor roads
VES Location
Local Councils Capital
Rivers
Footpath
6 00'
6 30'
4 30' 5 00'
10 Km0
Akata
Iyasan
Erinje
Igbisin Ebute Irele
Figure 6 a: Isopach Map of Surficial Aquifer around Okitipupa.
The Pacific Journal of Science and Technology –571– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
BIGHT OF BENIN
ODIGBO
OKITIPUPA IRELE
ESE - ODO
ILAJE
OmotosoIlebe
LokuboAbusoro
Agbetu
Erekiti
Ilado
Irowo
IgbotakoIju OkeLadenkehin
Ode Aye
Settlement
IgbolouyeForikuMulekangbo
Ilubirin
Agbabu
Igbaje
Oluagbo
IwadaIjuodo
IlutitunIkoyaOlotoMahintedo
Ode - IgbodigboIgboegunrin
Ebute - ErinjeAtijere
Ayesan
Aroromi Obu
Olowo
Lamudifa
OtuMkt
Olorunsogo
Odigbo
OREOfosu
Oja FagboOnisere
OrisunmibareOtugbembo
Poroye CampLodaAyadi
Legbagbo
Ladan Camp
Lejuwa
Laworo
Ebute Ipare
Kurawe
Fishing CampMahinUgbo
Ugbonla
Idi Ogba Ajapa
Oroto
Aiyetoro
Ashisha
Upe
IlepeteObe AduObenla
SekelewuOjumole
Aboto
IGBEKEBO
IGBOKODA
ShabomiIgbotu
OmiIRELEOKITIPUPA
Igodan
Old Ayeka
Araromi - Ayeka
Italuwa
Ago Ukgbene IpokaOjuala
Ajagba Salawa
Shegbemi
Gbeleju
Nagbore's Camp
Yopele
Madagbayo
AmapereArogbo
Uba - ArogboFiafio
Saforogbon
Okomu
Okorogbon
Ijaw Camp
Biagbene
Olowoso
Ayande
Gbu
Okoron ado
AkimoUpe
Gbelemotin
Soba Camp
Kajola
oke alafia
Oniparaga
Origbe
Lamira
Arijan
LEGEND
Major roads
Minor roads
VES Location
Local Councils Capital
Rivers
Footpath
6 00'
6 30'
4 30' 5 00'
10 Km0
Akata
Iyasan
Erinje
Igbisin Ebute Irele
Figure 6 b: Isopach Map of Intermediate Aquifer around Okitipupa.
The Pacific Journal of Science and Technology –572– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
This map shows that this unit thickens to the south in the area. The thickness of the intermediate aquifer on the other hand, ranges from 7.3m at Ebute Erinje to 80.7m around Ilutitun, with a mean value of 26.4m. However, the maximum profile length of AB = 650m adopted in this work could not delineate the intermediate aquifer in some sounding locations. Areas with thick aquifer unit may be good prospects for drilling boreholes with high yield expectations (Kelly, 1976; Mbonu et al., 1991). The isoresistivity maps for the area (Figures 7 a and 7 b) are derived from the resistivity parameters of the delineated aquifers. The upper aquiferous zone show relatively high resistivity ranging from 55 Ωm to 1374 Ωm, with a mean value of 504.7Ωm. These suggest lithology characterised by gravels and medium to coarse-grained sands (higher values), intercalated with fine-grained sands, silts and clay (lower values). In the intermediate aquifer, the resistivity range is 11-1001 Ωm, with a mean value of 296.8 Ωm. This value suggests better porosity and permeability and hence better groundwater saturation. Low resistivity values (11-55 Ωm) recorded around Aboto and Arogbo suggest saline or brackish water. Unconfirmed saline water intrusion into the coastal aquifers had earlier been reported in area (Omosuyi, 2001). Aquifer Potential and Vulnerability Potential Rating The assessment of the potentials of the surficial and intermediate aquifers in this study is based on aquifer thickness derived from VES data interpretation. The Quaternary coastal plain sands of the Dahomey basin around Okitipupa are generously porous and permeable (Okosun, 1998), important hydro-geologic parameters often synonymous with groundwater saturation. Studies (Ako, 1986: Omosuyi, 2001) have shown that the sands of the Coastal Plain around the area are usually characterised by high resistivity, deriving from the coarse, gravely nature, rather than a measure of groundwater saturation disposition. The VES-derived aquifer thickness in the upper and lower aquifer units have been used to zone the area into high (>30m), medium (10-30m), and
low (<10m) groundwater prospect zones as shown in Figure 8. CONCLUSION The results of surface geoelectric sounding around Okitipupa, south-western Nigeria, enabled the delineation of two shallow aquifer units within the Coastal Plain sands in the area. The upper/surfacial aquifer system occurs at depth ranging from 5.8 to 61.5m, with an average thickness of 29.5m and materials characterised by average resistivity of 504.7Ωm, suggesting gravely/coarse to medium sand. The top of the intermediate aquifer unit varies from 32.1 to 127.5m, with a mean thickness of 26.4m and relatively lower average resistivity of 296.8 Ωm, indicating medium to fine-grained sand and better groundwater saturation condition. Based on the resistivity parameters and overall thickness of geoelectric layers overlying the aquifer units, the near surface aquifer systems in the study area are generally protected from contaminants infiltrating from the surface, with the intermediate one being relatively less vulnerable. The integration of lithologic logs and other drilling data with surface geoelectric studies in the delineation of the aquifer systems and quantification of aquifer vulnerability in the area would enhance the accuracy of the results. However, this study is envisaged to provide guidance for drilling programmes, continuity for understanding the subsurface disposition of the shallow aquifer systems, and constitute a background information or useful guide for more elaborate groundwater development programme in the area. ACKNOWLEDGEMENTS Dr. M. I. Oladapo, Musa Bawallah, E. Ekundayo, S.Wahab, Dira Adegoke and Sam Ogunbowale assisted during data acquisition and graphic preparation. They are greatly appreciated.
The Pacific Journal of Science and Technology –573– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
BIGHT OF BENIN
ODIGBO
OKITIPUPA IRELE
ESE - ODO
ILAJE
OmotosoIlebe
LokuboAbusoro
Agbetu
Erekiti
Ilado
Irowo
IgbotakoIju OkeLadenkehin
Ode Aye
Settlement
IgbolouyeForikuMulekangbo
Ilubirin
Agbabu
Igbaje
Oluagbo
IwadaIjuodo
IlutitunIkoyaOlotoMahintedo
Ode - IgbodigboIgboegunrin
Ebute - ErinjeAtijere
Ayesan
Aroromi Obu
Olowo
Lamudifa
OtuMkt
Olorunsogo
Odigbo
OREOfosu
Oja FagboOnisere
OrisunmibareOtugbembo
Poroye CampLodaAyadi
Legbagbo
Ladan Camp
Lejuwa
Laworo
Ebute Ipare
Kurawe
Fishing CampMahinUgbo
Ugbonla
Idi Ogba Ajapa
Oroto
Aiyetoro
Ashisha
Upe
IlepeteObe AduObenla
SekelewuOjumole
Aboto
IGBEKEBO
IGBOKODA
ShabomiIgbotu
OmiIRELEOKITIPUPA
Igodan
Old Ayeka
Araromi - Ayeka
Italuwa
Ago Ukgbene IpokaOjuala
Ajagba Salawa
Shegbemi
Gbeleju
Nagbore's Camp
Yopele
Madagbayo
AmapereArogbo
Uba - ArogboFiafio
Saforogbon
Okomu
Okorogbon
Ijaw Camp
Biagbene
Olowoso
Ayande
Gbu
Okoron ado
AkimoUpe
Gbelemotin
Soba Camp
Kajola
oke alafia
Oniparaga
Origbe
Lamira
Arijan
LEGEND
Major roads
Minor roads
VES Location
Local Councils Capital
Rivers
Footpath
6 00'
6 30'
4 30' 5 00'
10 Km0
Akata
Iyasan
Erinje
Igbisin Ebute Irele
Figure 7a: Isoresistivity map of Surficial Aquifer around Okitipupa.
The Pacific Journal of Science and Technology –574– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
BIGHT OF BENIN
ODIGBO
OKITIPUPA IRELE
ESE - ODO
ILAJE
OmotosoIlebe
LokuboAbusoro
Agbetu
Erekiti
Ilado
Irowo
IgbotakoIju OkeLadenkehin
Ode Aye
Settlement
IgbolouyeForikuMulekangbo
Ilubirin
Agbabu
Igbaje
Oluagbo
IwadaIjuodo
IlutitunIkoyaOlotoMahintedo
Ode - IgbodigboIgboegunrin
Ebute - ErinjeAtijere
Ayesan
Aroromi Obu
Olowo
Lamudifa
OtuMkt
Olorunsogo
Odigbo
OREOfosu
Oja FagboOnisere
OrisunmibareOtugbembo
Poroye CampLodaAyadi
Legbagbo
Ladan Camp
Lejuwa
Laworo
Ebute Ipare
Kurawe
Fishing CampMahinUgbo
Ugbonla
Idi Ogba Ajapa
Oroto
Aiyetoro
Ashisha
Upe
IlepeteObe AduObenla
SekelewuOjumole
Aboto
IGBEKEBO
IGBOKODA
ShabomiIgbotu
OmiIRELEOKITIPUPA
Igodan
Old Ayeka
Araromi - Ayeka
Italuwa
Ago Ukgbene IpokaOjuala
Ajagba Salawa
Shegbemi
Gbeleju
Nagbore's Camp
Yopele
Madagbayo
AmapereArogbo
Uba - ArogboFiafio
Saforogbon
Okomu
Okorogbon
Ijaw Camp
Biagbene
Olowoso
Ayande
Gbu
Okoron ado
AkimoUpe
Gbelemotin
Soba Camp
Kajola
oke alafia
Oniparaga
Origbe
Lamira
Arijan
LEGEND
Major roads
Minor roads
VES Location
Local Councils Capital
Rivers
Footpath
6 00'
6 30'
4 30' 5 00'
10 Km0
Akata
Iyasan
Erinje
Igbisin Ebute Irele
Figure 7 b: Isoresistivity Map of Intermediate Aquifer around Okitipupa.
The Pacific Journal of Science and Technology –575– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
BIGHT OF BENIN
ODIGBO
OKITIPUPA IRELE
ESE - ODO
ILAJE
OmotosoIlebe
LokuboAbusoro
Agbetu
Erekiti
Ilado
Irowo
IgbotakoIju OkeLadenkehin
Ode Aye
Settlement
IgbolouyeForikuMulekangbo
Ilubirin
Agbabu
Igbaje
Oluagbo
IwadaIjuodo
IlutitunIkoyaOlotoMahintedo
Ode - IgbodigboIgboegunrin
Ebute - ErinjeAtijere
Ayesan
Aroromi Obu
Olowo
Lamudifa
OtuMkt
Olorunsogo
Odigbo
OREOfosu
Oja FagboOnisere
OrisunmibareOtugbembo
Poroye CampLodaAyadi
Legbagbo
Ladan Camp
Lejuwa
Laworo
Ebute Ipare
Kurawe
Fishing CampMahinUgbo
Ugbonla
Idi Ogba Ajapa
Oroto
Aiyetoro
Ashisha
Upe
IlepeteObe AduObenla
SekelewuOjumole
Aboto
IGBEKEBO
IGBOKODA
ShabomiIgbotu
OmiIRELEOKITIPUPA
Igodan
Old Ayeka
Araromi - Ayeka
Italuwa
Ago Ukgbene IpokaOjuala
Ajagba Salawa
Shegbemi
Gbeleju
Nagbore's Camp
Yopele
Madagbayo
AmapereArogbo
Uba - ArogboFiafio
Saforogbon
Okomu
Okorogbon
Ijaw Camp
Biagbene
Olowoso
Ayande
Gbu
Okoron ado
AkimoUpe
Gbelemotin
Soba Camp
Kajola
oke alafia
Oniparaga
Origbe
Lamira
Arijan
LEGEND
Major roads
Minor roads
VES Location
Local Councils Capital
Rivers
Footpath
6 00'
6 30'
4 30' 5 00'
10 Km0
Akata
Iyasan
Erinje
Igbisin Ebute Irele
Figure 8: Groundwater Prospect Map of Okitipupa Area.
The Pacific Journal of Science and Technology –576– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
REFERENCES 1. Ako, B.D. 1982. “Geophysical Prospecting for
Groundwater in an Area without Adequate Geological Data Base”. Journal of Mining Geology.
18(2):88-105. 2. Choudhry, K., Saha, D.K., and Chakrborty, P.
2001. “Geoelectrical Study for Saline Water Intrusion in a Coastal Alluvium Terrain”. Journal of Applied Geophysics. 46:189-200.
3. Dodds, A.R. and Ivic, D. 1988. “Integrated
Geophysical Methods used for Groundwater Studies in the Murray Basin, South Australia”. In: Geotechnical and Environmental Studies Geophysics, Vol II. Soc. Explor Geophys.:Tulsa,
OK. 303-310. 4. Ebraheem, A.M, Senosy, M.M., and Dahab, K.A.
1997. “Geoelectrical and Hydrogeochemical Studies for Delineating Ground-Water Contamination due to Salt-Water Intrusion in the Northern part of the Nile Delta, Egypt”. Ground Water. 35(2):216-222.
5. Edet, A.E. and Okereke, C.S. 2002. “Delineation of
Shallow Groundwater Aquifers in the Coastal Plain Sands of Calabar Area (Southern Nigeria) using Surface Resistivity and Hydrogeological Data”. Journal of African Earth Sciences. 35:433-443.
6. El-Waheidi, M.M., Merlanti, F., and Paven, M.
1992. “Geoelectrical Resistivity Survey of the Central Part of Azraq Basin (Jordan) for Identifying Saltwater/Freshwater Interface”. J. Applied Geophysics. 29:125-133.
7. Fetter, C.W. 1980. Applied Hydrogeology. C.E.
Merrill Publishing: Columbus. OH. 306-370. 8. Freeze, R.A. and Cherry, J.A. 1979. Groundwater.
Prentice-Hall: Englewood, NJ. 604. 9. Frohlich, R.K., Urish, D.W., Fulter, J., and Reilly,
M.O. 1994. “Use of Geoelectrical Method in Groundwater Pollution Surveys in a Coastal Environment”. Journal of Applied Geophysics. 32: 139-154.
10. Iloeje, M.P. 1981. A New Geography of Nigeria.
Longman: Nigeria. 26 – 28. 11. Jones, H.A. and Hockey, R.D. 1964. “The Geology
of Part of Southwestern Nigeria”. Geol. Surv. Nigeria Bull. 31:87.
12. Kalinski, R.J., Kelly, W.E., and Bogard, S. 1993.
“Combined Use of Geoelectrical Sounding and Profiling to Quantify Aquifer Protection Properties”. Ground Water. 31:538-544.
13. Kehinde, M.O. and Loehnert, F.P. 1989. “Review
of African Groundwater Resources”. Jour. of African Earth Science. 9(1):179-185.
14. Keller, G.V. and Frischnecht, F.C. 1966. Electrical
Methods in Geophysical Prospecting. Pergamon
Press: Oxford, UK. 523. 15. Kelly, E.W. 1976. “Geoelctric Sounding for
Delineating Ground Water Contamination”. Ground Water. 14:6-11.
16. Kirsch, R., Sengpiel, K.P., and Voss, W. 2003.
“The Use of Electrical Conductivity Mapping in the Definition of Aquifer Vulnerability Index”. Near Surface Geophy. 1:13-20.
17. Koefeod, O. 1979. Geosounding Principles, 1.
Resistivity Sounding Measurements. Elsevier
Scientific Publishing Comp.: Amsterdam, The Netherlands. 275.
18. Lashkarripour, G.R. 2003. “An Investigation of
Groundwater Condition by Geoelectrical Resistivity Method: A Case Study in Korin Aquifer, Southeast Iran”. Journal of Spacial Hydrology. 3(1):1-5.
19. Leite, J.L. and Barker, R.D. 1978. “Resistivity
Surveys Employed to Study Coastal Aquifers in the State of Bahia, Brazil”. Geoexploration. 16:
251-257. 20. Mbonu, P.D.C., Ebeniro, J.O., Oeoegbu, C.O.,
and Ekine, A.S. 1991. “Geoelectric Sounding for the Determination of Aquifer Characteristics in Parts of the Umuahia Area of Nigeria”. Geophysics. 56(2):284-291.
21. Okosun, E.A. 1998. “Review of the Early Tertiary
Stratigraphy of Southwestern Nigeria”. J. of Mining and Geology. 34:27-35.
22. Omosuyi, G.O., Ojo, J.S., and Olorunfemi, M.O.
1999. “Borehole Lithologic Correlation and Aquifer Delineation in Parts of the Coastal Basin of SW Nigeria”. Journal of Applied Sciences. 2:617-626.
23. Omosuyi, G.O. 2001. “Geophysical and
Hydrogeological Investigations of Groundwater Prospects in the Southern part of Ondo State, Nigeria”. Ph.D. Thesis, Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria. 195.
24. Omotsola, M.E. and Adegoke, O.S. 1981.
“Tectonic Evolution and Cretaceous Stratigraphy of the Dahomey Basin”. J. Min. Geol. 18(1):130-137.
The Pacific Journal of Science and Technology –577– http://www.akamaiuniversity.us/PJST.htm Volume 9. Number 2. November 2008 (Fall)
25. Orellana, E. and Mooney, H.M. 1966. “Master Tables and Curves for Vertical Electrical Sounding over Layered Structures”. Inteciencis, Madrib. 34.
26. Schwarz, S.D. 1988. “Application of Geophysical
Methods to Groundwater Exploration in the Tolt River Basin, Washington State”. In: Geotechnical and Environmental Studies Geophysics, Vol II. Soc. Explor Geophys.: Tulsa, OK. 213-217.
27. Thorling, L., Sorensen, K.I., and Ernstsen, V. 1997.
“Effect of “Windows” Mapped by Geophysical Methods on Groundwater Quality”. Proceedings Environmental and Engineering Geophysical Society European Section, EEGS-ES. 9-12.
28. Vander-Velpen, B.P.A. 1988. “RESIST Version
1.0.” M.Sc. Research Project. ITC: Delft Netherlands.
29. Van Overmeeren, R.A. 1989. “Aquifer Boundaries
Explored by Geoelecrtrical Measurements in the Coastal Plain of Yemen: A Case of Equivalence”. Geophysic. 54(1):38-48.
30. Van Stempvoort, D., Ewet, L., and Wassenaar, L.
1993. “Aquifer Vulnerability Index: A GIS-Compatible Method for Groundwater Vulnerability Mapping”. Canadian Water Resources Journal. 18: 25-37.
31. Zohdy, A.A.R. 1965. “The Auxiliary Point Method of
Electrical Sounding Interpretation and its Relationship to Dar Zorrouk Parameters”. Geophysics. 30:644-650.
SUGGESTED CITATION Omosuyi, G.O., J.S. Ojo, and M.O. Olorunfemi. 2008. “Geoelectric Sounding to Delineate Shallow Aquifers in the Coastal Plain Sands of Okitipupa Area, Southwestern Nigeria”. Pacific Journal of
Science and Technology. 9(2):562-577.
Pacific Journal of Science and Technology
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